[leetcode][125]Valid Palindrome

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import string
class Solution(object):
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        valid = set(string.ascii_lowercase+ string.digits)
        
        left = 0
        right = len(s) -1
        while left < right:
            s_l = s[left].lower()
            s_r = s[right].lower()
            
            if (s_l in valid) and (s_r in valid):

                if s_l == s_r:
                    left += 1
                    right -= 1
                else:
                    return False
                    
            elif s_l in valid:
                right -= 1
            elif s_r in valid:
                left += 1
            else:
                left += 1
                right -= 1
        return True